Multistage heating plants have been in use for many years, and typically are made up of heating equipment that provides heat from two different sources. Examples of such multistage heating plates are plants which include heat pumps with auxiliary electric resistance heat, heat pumps with auxiliary gas furnaces, and solar energy sources with auxiliary heat of either electric or fossil fuels. The reason for using heating plants that have primary and secondary fuel sources is to take advantage of a lower cost heating fuel with the use of the primary source, and then utilizing the secondary source which requires a more costly fuel.
In recent years, to increase the cost efficiency of the use of multistage equipment, it has been common to install outside temperature sensing equipment to aid in the staging of the primary and secondary heating plants. As is well known, it is more efficient to provide heat by operating a heat pump than by the use of a secondary heating source such as electric heat. As this is the case, it is desirable to prevent the electric heat from becoming operable if the primary source, the heat pump, can handle the heating load by itself. Where a multistage system is operated at a constant indoor temperature, this creates no significant problem. If the indoor temperature is held constant, the natural variations in outdoor temperature cause the heat pump and its auxiliary heat to be staged in its most efficient manner. That is, the heat pump will supply heat until it is no longer able to maintain a stable temperature and then the auxiliary or electric heat will be brought into operation. A drawback, however, even with constant setpoint conventional multistage thermostats is the excessive offset or droop required to initiate operation of the auxiliary heating stages. To enhance comfort, outdoor sensors are frequently applied to reset or dedroop the thermostat control point. The effectiveness of outdoor reset depends on matching the reset means to the heating system capacity and the design weather conditions.
In order to save energy, it has become commonplace to provide a night setback function to many thermostats along with its associated day setup function. These functions provide for a lower nighttime temperature within the heated building, and a morning pickup or return to the normal daytime ambient temperature. Where night setback and morning pickup are provided, a heat pump may not be operated in its most efficient mode. For example, in the morning when the thermostat is returned to normal daytime ambient temperature control, the heat pump is turned on along with a number of stages of electric heat. This morning pickup therefore utilizes both the primary and secondary heating sources at the same time regardless of whether or not the heat pump alone might be capable of bringing the temperature of the building up to the setpoint temperature if given sufficient time. In order to prevent this type of a system from having the electric heat staged on at the time of morning pickup, many installations contain outside thermostats which lock the electric heat off if the outdoor ambient is above some predetermined temperature. This type of system is expensive and inefficient. It is expensive from the point of view that it requires an outside temperature sensing device and auxiliary control equipment, along with the loss of efficiency as many installers misadjust the equipment so that electric heat is brought on even though it would not be required to bring the building up to the normal daytime ambient. Moreover, the outdoor temperature at which the heat pump alone is capable of meeting the building heating demand varies depending upon solar, wind, and internal heat (occupants, lights, etc.) gains and losses. These effects can render even a properly adjusted outside thermostat inefficient.
In summary, it is common in the control of heat pumps with auxiliary heat, particularly electric heat, to install outside sensing equipment that reduces control point offset (droop) and overrides effects of setpoint changes in setup in the morning setback at night. The installation of this extra equipment, and the deficiencies in its adjustment and effectiveness, make the operation of heat pump equipment and auxiliary heat marginal in many types of installations. In some localities, the electric utilities supplying power have suggested that heat pump installations be operated without night setback and morning pickup just because of the conditions recited above.